1. Field of the Invention
The present invention relates to a discharge recording medium with good environmental resistance and capable of good printing performance, on which information is printed by breaking apart a metal thin layer formed on a substrate layer by electric discharge and heat to perforate the thin metal layer.
2. Description of the Related Art
Heretofore, as one printing method which prints information having good environmental resistance, a thermal printing method has been used in which, a part of a thin layer of a low melting point metal is heated until molten to perforate the metal layer and print information.
Since a recording medium printed by the thermal printing method is not rewritable and has good long-time stability, this method is widely used to record money amounts, numbers or dates.
An example of the thermal printing method is disclosed in Japanese Patent Publication No. 19928/1984. In this patent, a magnetic recording layer is formed on a substrate and then a thin layer of a non-magnetic metal or alloy is formed, as a thermal recording layer, on the magnetic recording layer. Various modifications are also disclosed. For example, a colored layer may be provided between the thermal recording layer and the magnetic recording layer, or a colored layer and a protective layer may be provided on the thermal recording layer, or an adhesive layer may be provided between each pair of adjacent layers.
Since the thermal printing method records information by heating and cooling a thermal head, its printing speed is limited so that the use of this method is very much limited where high speed processing is required, for example in transportation, stations and supermarkets.
Another printing method which uses melting of a thin metal layer is a discharge printing method which is disclosed, for example, in Japanese Patent Publication No. 30157/1983 and U.S. Pat. No. 4,400,706. The information printed by this method is not rewritable either as in the case of the thermal printing method, and it has the advantage that high speed printing is possible.
A recording medium used in the discharge recording method, namely a discharge recording medium, has poorer environmental resistance than a recording medium used in the thermal printing method. In addition, by the discharge recording method, it is difficult to print clear images unless a high electric field is applied between a recording stylus and the thin metal layer. This is because the principle of the discharge recording method is based on the perforation of the metal layer by melting and splashing the metal from the part of the metal layer to be perforated, so that thickness of a protective layer cannot be made thin.
In addition, the discharge recording method is unsatisfactory in its recording density and may provide unclearer images than the thermal recording method. This is partly because the discharge breakdown of the thin metal layer is caused only in a part of a printing dot through which an electric current passes so that it is difficult to form a complete printing dot and partly because a condition of electric current from the recording stylus tends to be affected by a delicate change of the thickness of the protective layer on the metal layer.
As is clear from the printing mechanism, a clear image is produced only by the application of high energy such that a high electric field is generated between the recording stylus and the metal layer. To this end, there should be a certain resistance between the recording stylus and the metal layer. However, too high of a resistance will prevent passage of the electric current. The resistance cannot be completely controlled by the change of the thickness of the protective layer. That is, since the thickness of the protective layer is at most 1 .mu.m, the thickness of the protective layer formed by a conventional coating means such as a gravure coater, will fluctuate within .+-.20%. At a part having a larger thickness, the electric resistance increases and passage of the electric current is difficult so that the printing density decreases, thus resulting in dispersion of the printing density.
It may be possible to decrease the dispersion of the printing density by a decrease of the thickness of the protective layer, but this will decrease the environmental resistance of the recording medium. As explained above, the discharge recording medium has a poor environmental resistance and low printing density.
When the discharge recording medium has a poor environmental resistance and is exposed to a harsh atmosphere, for example, a high humidity atmosphere, the thin metal layer will corrode so that its appearance is changed or a part of the metal layer drops off, whereby the printed parts disappear or fade. When the printing density of the discharge recording medium is low, visibility of the printed information is worsened, the printing quality deteriorate and reliability of the recording is lowered.